Thermal efficiency and engine-out emissions from an internal combustion engine are determined by many factors including the combustion chamber shape, the fuel injection nozzle, fuel injection pressure, to name a few. Much is known and much has been studied in typical diesel engine combustion chambers. However, in unconventional engines, less is known about what combustion chamber shape and fuel injection characteristics can provide the desired performance.
Such an unconventional engine, an opposed-piston, opposed-cylinder (OPOC) engine 10, is shown isometrically in FIG. 1. An intake piston 12 and an exhaust piston 14 reciprocate within each of first and second cylinders (cylinders not shown to facilitate viewing pistons). An intake piston 12′ and an exhaust piston 14 couple to a journal (not visible) of crankshaft 20 via pushrods 16. An intake piston 12 and exhaust piston 14′ couple to two journals (not visible) of crankshaft 20 via pullrods 18. The engine in FIG. 1 has two combustion chambers formed between a piston top 22 of intake piston 12 (or 12′) and a piston top 24 of exhaust piston 14 (or 14′) and the cylinder wall (not shown). The pistons in both cylinders are shown at an intermediate position in FIG. 1. Combustion is initiated when the pistons are proximate each other. The piston tops 22 and 24 in FIG. 1 may not be optimized to provide the desired performance. The piston top 24 has a raised region at the periphery and a flat bowl in the middle of the chamber. To achieve a desired compression ratio, the volume contained in the piston bowls is prescribed. Piston top 24 has a raised region, known by one skilled in the art as squish. The projected area of the squish region is a small portion of the projected area of piston top 24, whereas the bowl is the greater portion of the projected area. Because of the large area taken up by the bowl, the depth of the bowl is limited. Such a shallow bowl allows little space to accommodate fuel jets from an injector to enter the combustion chamber without significantly impinging on piston top surfaces.